[feat]: add support for page.waitForSelector() (#1509)

# why
- adds support for a new function: `page.waitForSelector()`, which works
with cross-iframe selectors & cross-shadow root selectors
# what changed
- added `page.waitForSelector()` which returns `true` when the element
that the selector points to resolves, and times out with an error when
it does not resolve
# test plan
- added a bunch of e2e test cases, covering various CSS selector types,
& cross iframe/shadow root cases
<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds Page.waitForSelector() to wait for CSS or XPath targets to reach a
state, piercing shadow DOM and traversing iframes. This makes waits
reliable across complex trees and uses MutationObserver for efficiency.

- **New Features**
- Page.waitForSelector(selector, { state, timeout, pierceShadow }) with
states: attached, detached, visible, hidden (defaults: visible, 30s,
pierceShadow=true).
- Supports CSS and XPath (xpath= or /), including open and closed shadow
DOM via the V3 piercer.
- Works across iframes and with '>>' hop notation; resolves the correct
frame automatically.
  - Clear timeout errors; no polling (MutationObserver-based).

- **Refactors**
- Added resolveLocatorTarget to unify hop/XPath frame resolution and
reuse in waitForSelector.
  - Exported waitForSelector locator script under dom/locatorScripts.

<sup>Written for commit a238850.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

---------

Co-authored-by: tkattkat <tkat@tkat.net>

Author: seanmcguire12

.changeset/silly-rooms-grow.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+add support for page.waitForSelector()

packages/core/lib/v3/dom/locatorScripts/index.ts

@@ -1,3 +1,4 @@
 export * from "./scripts";
 export * from "./selectors";
 export * from "./counts";
+export * from "./waitForSelector";